The present invention relates to an extended nip press for dewatering a pulp web in a paper-making machine which comprises two press rolls that form a press nip, the first press roll being retained in the axial direction at least at one end, and having first bearing blocks to hold first bearing pins of the first press roll, and having second bearing blocks to hold second bearing pins of the second press roll, and having tension elements by means of which the second bearing blocks can each be tensioned with respect to the first bearing blocks, the tension elements allowing a relative displacement of the press rolls in the axial direction.
An extended nip press of this kind is known from WO 92/17641.
In the known press apparatus, two press rolls, between which a press nip is formed, are arranged parallel to one another.
Since the first bearing blocks and the second bearing blocks are tensioned with respect to one another by means of tension elements, the result is a short force flow path for transfer of the pressing force in the press nip which does not stress any of the frame parts. The frame must therefore transfer only the dead weight of the press, but not the high pressing forces. The result is therefore a simpler, lighter, and more space-saving construction. In the known press apparatus, the tension elements consist of a center part similar to a leaf spring, and hammer-heads at the ends which are held in grooves on the bearing blocks. The bearing blocks are thus connected directly by means of the aforesaid tension elements. The tension elements, which are flexurally elastic in the axial direction of the press rolls, thus allow mutual deflection of the press rolls and a certain mutual axial displaceability of the press rolls during operation due to high pressing forces or due to changes in length which may, for example, be caused by temperature. When the press apparatus is unloaded, the tension elements are preloaded either not at all or only very slightly.
In the known apparatus, the first press roll is configured as a deflection compensated roll, i.e. the roll possesses a stationary supporting member or a yoke on which is rotatably mounted a roll shell that is hydraulically braced on the yoke, so that even when extraordinarily high pressing forces are present during operation, the roll shell can be adjusted to have practically deflection-free characteristics or even, if desired, to have a specific deflection, the yoke being capable of deflecting. At the same time, this considerably simplifies the construction of the bearings. In the known arrangement, the second press roller is configured as a shoe press roll, which again has a stationary supporting member over which a tubular pressing shell circulates. In the region of the press nip, this pressing shell runs over a press shoe which is adapted to the shape of the opposing roll, i.e. the first press roll which is embodied as a deflection compensated roll, thereby forming an extended press nip. Thus, an extraordinarily high pressing force can be generated in the region of the press nip, and at the same time a gradual increase in pressure upon entry into the press nip is possible.
When the width of the pulp web is smaller, in many cases the deflection compensated roll is replaced by a quasi-deflection-free solid roll, since the load is lower.
However, it has been found that in certain cases the tension elements of this pior art design may be subject to premature failure.